(1) Field of the Invention
This invention relates to a method for producing a thermoplastic resin sheet or a filler-containing resin sheet.
More particularly, the invention relates to a method for producing a thermoplastic resin sheet or a filler-containing resin sheet which is especially suitable for producing a sheet made of thermoplastic resins such as ultra-high molecular weight polyethylene and polytetrafluoroethylene that are difficult to process due to their high melt viscosities and the narrowness between their melting points or softening points and thermal decomposition temperatures, and a resin sheet that contains a large quantity of a filler, or a special class of fillers that will be described later.
(2) Description of the Prior Art
In the conventional art, ordinary plastic films and sheets of polyethylene or polypropylene are produced by tubular film extrusion method of T-die method. These methods are widely employed in the industry because the methods have advantages that they can be put into practice with simple equipment and uniform products can be obtained at high productivity.
The above forming methods, however, cannot be employed for the processing of difficultly-processable resins such as ultra-high molecular weight polyethylene and polytetrafluoroethylene as described below.
Meanwhile, in order to impart desirable properties to thermoplastic resins, fillers are often compounded into resin materials. In the above processing methods, however, when a filler is compounded in large quantities, the forming operation becomes difficult because their flow properties such as melt flow become worse.
Furthermore, when fibrous fillers such as glass fiber and carbon fiber, hollow particle fillers such as glass balloons, and flaky fillers such as mica are used, the abrasion of the apparatus is accelerated and quite a large cost for the maintenance of extruder is necessary. In addition, the obtained products cannot have the desired properties because particles of the filler are broken down by the shearing force caused in the extrusion.
Ultra-high molecular weight polyethylene has excellent impact strength and abrasion resistance. However, the consumption of this resin is very small because the processing of a resin of this kind is difficult. The methods for forming articles of ultra-high molecular weight polyethylene are exemplified by sintering, ram extrusion using a plunger pump, and forging. For forming plates and boards from this resin, compression molding is mainly employed. However, it is very difficult to produce thin sheets or films of less than 1 mm in thickness from ultra-high molecular weight polyethylene. (The term "sheet or film" will be hereinafter referred to as simply "sheet" and it should be noted that the term "sheet" or "sheets" used also in connection with the method of this invention includes "film(s) and sheet(s)". According to the present industrial practice, thin sheets of this kind are produced by applying a secondary working such as skiving to a cylindrical body of material that is obtained by the abovementioned method. Therefore, in the above method, the processing cost is high as well as the continuous mass production of thin sheets is quite difficult.
This fact depends upon the reason that ultra-high molecular weight polyethylene has quite a high melt viscosity and is not good in melt flow, and therefore, the conventionally employed tubular film method and T-die method in which the pressure drop is large, are not suitable.
In the case of polytetrafluoroethylene, since the difference between its forming temperature and thermal decomposition temperature is small, the above ordinary forming methods cannot be employed, either.
Meanwhile, as a hitherto well known method for forming sheets, there is a calender roll method. Because sheets of accurate and uniform thickness can be produced at a high rate, polyvinyl chloride sheets and rubber sheets are produced by this method. In the case of polyolefin resins, their melt strengths are low and their melt viscosities are largely dependent on temperatures, so that the optimum range of their forming temperatures is narrow, which makes the forming operation difficult. Accordingly, this method is seldom employed practically.